The Gilbert universal motors may run very slightly quieter on DC but never cooler.
Back to the original comment about the "AC" suffix on some engine numbers. In 1949 Gilbert began adding the "AC" suffix to engines and for the most part the tender lettering on "AC" engines changed to sans serif. Probably someone in design or marketing advanced this idea, there was no significant internal change in the engines that drove it. In 1952 with the introduction of knuckle couplers on two engines (Hudson and Northern) those two engines were numbered with a "K" prefix and the "AC" suffix was dropped (a 325AC became a K325). In 1953 with the wide use of knuckle couplers the "AC" suffix was dropped from use and the KC engines were up numbered so a K325 was now a 326. Serif tender lettering also reappeared on KC engines and "American Flyer" tenders again became "American Flyer Lines" tenders.
The last DC motored engines were made in 1950, from 1951 onward all Gilbert engines had universal motors so the "DC" suffix was not needed.
The "DC" suffix was not used for most DC motored engines. Most were just numbered as 332's and 342's but with DC motors. When purchasing a 332 or 342 it is necessary to look at the motor to see if it is a DC or a universal motor.
